Motor vehicles driven by a plurality of axles can be divided into those with a hang-on four-wheel drive wherein a primary axle is driven permanently and a secondary axle is connected if required; and those with a permanent four wheel drive wherein both axles are driven permanently. The design of the driveline is largely influenced by the arrangement of the engine in the motor vehicle, i.e. whether it is arranged in the front or at the rear and whether it is positioned in the longitudinal or transverse direction. At the same time, stringent packaging requirements exist regarding size, weight, and assembly costs of such systems.
In a typical front-wheel-drive based all-wheel-drive hang on layout, the engine is transversely mounted, and the front differential is included as part of the transmission assembly. The vehicle can be driven in an all-wheel-drive configuration by taking power off of the power takeoff unit and delivering it to the rear wheels. This can be accomplished by several known couplings, including on-demand couplings.
In vehicles where it is desirable to provide a very low range (creep) drive mode, however, the vehicle transmission may not provide a low enough drive ratio to enable a very low range drive ratio. This may be particularly true in cases of conventional transmission designs.
Thus, in front-wheel-drive based all-wheel-drive systems, there exists a need for reducing the drive speed of the power takeoff unit to enable a low range drive ratio.